1. Field of the Invention
The invention relates to a receiver in a radio system, and especially to error correction during signal reception.
2. Description of the Related Art
As the number of users of radio systems increases, radio systems need to be positioned close to each other in the frequency domain so as to be able to utilize the available frequencies as efficiently as possible. For the reception of signals, the closeness of the systems is disadvantageous, because the demands on signal filtering, A/D conversion, error correction and interference elimination increase. For instance in A/D conversion, an interference signal that is multifold in strength with respect to the signal to be detected can cause saturation in the A/D converter, which can be prevented by changing the amplification of the signal. FIG. 1 shows the spectrum of a signal received in a radio system, with the frequency on a horizontal axis and the magnitude spectrum in decibels on the vertical axis. The central frequency of the signal to be detected 100 is approximately 2 MHz, whereas the central frequency of the interference signal 102 that is multifold with respect to the signal to be detected is approximately 5 MHz. If the receiver was a narrowband receiver, the interference signal 102 could be filtered before amplification adjustment and A/D conversion. The received band is very broad in broadband receivers and if the received band contains interference components, their analog filtering is not possible. In a broadband receiver, the interference signal is separated from the desired signal after the A/D conversion.
FIG. 1 shows that the power level of the desired signal is very low, so, in practice, the signal should be amplified before the analog-to-digital conversion. FIG. 2 shows a signal spectrum after a +6-dB amplification step. The figure shows how the amplification step has degraded the signal spectrum to a broader frequency range, which is a result of the non-linear nature of the amplification change. Degrading the signal spectrum is harmful for the detection of the desired signal, because the desired signal can no longer be easily separated from the broad spectrum. In the same manner as the amplification change, filtering also causes the degradation of the signal spectrum. In a filter, the degradation of the spectrum is caused by the effect of the step response of the filter.
Reference publication EP 1,129,522 describes a known solution for correcting amplification. In known solutions, the effect of the step response of a filter is reduced by deleting a few samples around the amplification step and by replacing the deleted samples by interpolated samples. The steeper the filter, the more samples need to be interpolated in practice. It is clear that a very good performance in signal reception cannot be achieved with the interpolation of samples, if there are several interpolated samples one after the other.